A wireless local area network (WLAN) typically includes an Access Point (AP) and one or more stations. Each station may be a device such as a notebook computer, personal digital assistant (PDA), wireless VoIP telephone or the like that transmits radio signals to and receives radio signals from other stations in the local area network via the AP.
APs and stations transmit data in units referred to as frames over a shared-communications channel. Frames transmitted from a station to an AP are referred to as uplink frames, and frames transmitted from an AP to a station are referred to as downlink frames. In a situation where two or more stations (or an AP and a station) transmit frames simultaneously, then one or more of the frames can become corrupted, referred to as a collision. As a result, Wireless Local Area Networks (WLANs) typically employ one or more protocols to ensure that a station or AP can gain exclusive access to the shared-communications channel for a predetermined interval of time in order to transmit its frames without collisions occurring. Certain wireless network protocols (e.g., Institute of Electrical and Electronics Engineers [IEEE] 802.11, etc.) provide for the AP to periodically broadcast a special frame called a beacon that can be heard by the stations in the BSA (basic service area), the area covered by the AP. The beacon contains a variety of information that enables the stations to establish and maintain communications in an orderly fashion, such as a timestamp, which enables stations to synchronize their local clocks, and signaling information (e.g., channel number, frequency hopping pattern, dwell time, etc.).
A station can prolong its battery life by powering off its radio when not transmitting or receiving. When a station powers off its radio, the station is said to enter the “doze” state. A station wakes up from the doze state by powering on its radio to enter the “awake” state. While a station is in the doze state, it cannot transmit or receive signals, and is said to be asleep. A station that saves battery life by alternating between alert states and doze states is said to be in power save (PS) mode, and a station that employs PS mode is said to be a power saving station.
While a station is asleep, the AP buffers any downlink frames for the station for eventual delivery when station is in the awake state.
One method for receiving buffered frames is described in the IEEE 802.11-1999 standard, and is referred to herein as the “legacy” power save method. In this method, an AP periodically includes a Traffic Indication Map (TIM) in the beacon to identify which stations in power-save mode have downlink frames waiting in the AP's buffer for transmission.
In accordance with legacy power save method, stations in the doze state wake up to receive beacons and check the TIM. If the TIM indicates that there are frames buffered for the station, the station will send a PS Poll to request delivery of a buffered frame. When the TIM indicates that there are no buffered downlink frames for a station, the station goes back into the doze state. In addition, a station in the doze state buffers uplink frames generated by the application layer, and transmits one or more of the buffered uplink frames upon wake-up.
Another strategy for delivering buffered packets in the AP to the appropriate station is known as Automatic Power Save Delivery (APSD), wherein the delivery of downlink buffered frames occurs without the need for PS Polls to indicate that the PS station is awake and ready to receive transmitted frames. There are two types of APSD: (i) scheduled APSD and (ii) unscheduled APSD. The two APSD variants differ with respect to initiation of the ‘service period’, which is the time period during which the PS station is presumed awake and able to receive a frame transmitted by the AP. With scheduled APSD, the service period starts automatically—that is, without special signaling frames to notify the AP that a station is awake and ready to receive frames. With unscheduled APSD, the transmission of an uplink frame is sufficient to signal the start of the service period, i.e., that the station is awake and ready to receive transmitted frames. The unscheduled service period begins when the AP receives from the station a QoS Data/Null frame. The station may elect to limit the access category (AC) of the frame that may initiate a service period. The AC(s) of the frames that may initiate a service period are designated by the station to through signaling such as an APSD TSPEC
Another feature of APSD relates to the termination of the service period, the time interval a PS station must remain awake. Unlike legacy power save, where a PS station may go to sleep after receiving a single frame from the AP, APSD requires a PS station to stay awake to receive several buffered frames, and go to back to sleep only when it is notified by the AP. The AP sets the EOSP (end of service period) bit to 1 in the last frame it transmits in order to signal to the station that it will not transmit any more frames downlink until the next service period. This signals the station that it can go back to sleep.